1. Field of the Invention
The present invention relates to an actuator which is suitably mounted as a drive source for a small electronic device such as a camera, a light quantity adjusting apparatus using the actuator, and a stepping motor to which principle of the actuator is applied.
2. Related Background Art
FIG. 18 shows a conventional shutter apparatus for a lens shutter camera.
In FIG. 18, reference 101 denotes a permanent magnet, 102 denotes a driving lever, and 102a denotes a driving pin provided in the driving lever 102. The driving lever 102 is fixed to the permanent magnet 101 and integrally rotates with the permanent magnet 101. Reference numeral 103 denotes a coil and 104 and 105 denote stators which are made of a soft magnetic material and excited through the coil 103. The stator 104 and the stator 105 are connected with each other through portions 104a and 105a, so that the stator 104 and the stator 105 are integrally formed in view of a magnetic circuit. When a current is supplied to the coil 103, the stator 104 and the stator 105 are excited, with the result that the permanent magnet 101 rotates within a predetermined angle range. Reference numerals 106 and 107 denote shutter blades and 108 denotes a base plate having an opening portion 108a. Hole portions 106a and 107a of the shutter blades 106 and 107 are rotatably fit to pins 108b and 108c of the base plate 108. The driving pin 102a is slidably inserted into long holes 106b and 107b of the shutter blades 106 and 107. When the driving lever 102 rotates with the permanent magnet 101, the shutter blades 106 and 107 pivot about the hole portions 106a and 107a, so that an aperture which is not shown is opened or closed.
As another mode, there is also a shutter apparatus having a structure in which a permanent magnet is made of plastic and integrally formed with a driving pin in order to prevent an increase in cost.
Reference numeral 109 denotes a front base plate for holding the shutter blades 106 and 107 so that the blades are movable between the front base plate 109 and the base plate 108. Reference numeral 110 denotes a rear base plate for holding the stators 104 and 105 and holding the permanent magnet 101 so that the magnet is rotatable.
The use of a digital camera has been widely spread. The digital camera performs photoelectric conversion on an image of a field to be imaged using a CCD or the like as an image pickup device and causes a recording medium to record a converted image as still image information. The operation of the digital camera of this type with respect to exposure will be briefly described below.
First, a main power source is turned on before photographing. When the image pickup device becomes an operating state, the shutter blades are held at an open position in which the image pickup device can be exposed. Therefore, the accumulation of charge and the transfer (emission) thereof are repeated in the image pickup device, with the result that the field to be imaged can be observed through an image monitor.
After that, when a release bottom is pressed, a diaphragm value and an exposure time are determined according to an output from the image pickup device at this time. Then, whether or not it is necessary to narrow the diameter of an exposure aperture is determined based on determined results. When it is necessary to narrow the diameter of the exposure aperture, the shutter blades are driven to set a predetermined diaphragm value. Next, an instruction for starting the accumulation of charge (accumulation start signal) is provided to the image pickup device from which the accumulated charge has emitted. Simultaneously, an exposure time control circuit is activated in response to the accumulation start signal serving as a trigger signal. After a lapse of a predetermined exposure time, the shutter blades are driven to a closed position in which exposure to the image pickup device is blocked. After the exposure to the image pickup device is blocked, the accumulated charge is transferred and image information is recorded on a recording medium through an image writing device. The blocking of exposure to the image pickup device during the transfer of charge is intended for preventing a charge quantity from changing by unnecessary light during the transfer of charge.
In addition to the above-mentioned shutter apparatus, there are a shutter apparatus having a mechanism for inserting and removing an ND filter into and from an optical path and a shutter apparatus having a mechanism for inserting and removing a diaphragm regulation member having a small diaphragm diameter into and from an optical path.
In the conventional shutter apparatus, a coil and stators take up a great deal of base plate space. Therefore, it is hard to locate another actuator, a guide rod for a lens, and the like. In view of this point, the following light quantity adjusting apparatus is proposed in Japanese Patent Application Laid-open No. 2002-049076 of the application filed by the present applicant.
FIG. 19 is an exploded perspective view showing a light quantity adjusting apparatus disclosed in Japanese Patent Application Laid-open No. 2002-049076. FIG. 20 is a sectional view showing the light quantity adjusting apparatus shown in FIG. 19 in an axial direction. The light quantity adjusting apparatus includes an actuator serving as a driving device. The actuator has a magnet 201, a coil 202, a stator 203, an auxiliary stator 204, and a blade driving pin 201h. The magnet 201 is rotatable around the center of rotation as an axis and at least the outer peripheral surface thereof is divided in a peripheral direction so as to be alternately magnetized in different poles. The coil 202 is located in the axial direction of the magnet 201. In the stator 203, an outside magnetic pole portion 203a and an inside magnetic pole portion 203b which are excited by the coil 202 are opposed to the outer and inner peripheral surfaces of the magnet 201. The auxiliary stator 204 is fixed to the inside magnetic pole portion 203b of the stator 203 and excited by the coil 202. The blade driving pin 201h is integrally formed with the magnet 201. The light quantity adjusting apparatus further includes a base plate 205 with an opening portion 205a and light quantity control blades 207 and 208. The light quantity control blades 207 and 208 are driven by the blade driving pin 201h of the actuator so as to adjust an opening quantity of the opening portion 205a of the base plate 205. Reference numeral 206 denotes a retaining plate.
When the light quantity adjusting apparatus having the above-mentioned structure is used, the coil and the magnet are located in the axial direction. Therefore, there is a large effect that the light quantity adjusting apparatus becomes a compact apparatus in which the coil and the magnet do not take up a great deal of base plate space.
When an additional reduction in size of a digital camera or the like using the above-mentioned actuator is demanded, it is necessary to achieve slimming of the digital camera in addition to a reduction in size of the actuator itself. However, in the above-mentioned actuator, the coil and the magnet are located in the axial direction. Therefore, extreme slimming is difficult because efficiency of the actuator is likely to reduce owing to a reduction in magnetization efficiency of the magnet in the peripheral direction.
On the other hand, the fundamentals of an actuator having a rotating shaft are also applied to a stepping motor.
FIG. 21 is a partially sectional view showing a two-phase stepping motor disclosed in Japanese Patent Publication No. H06-083561 as an example of a conventional technique. The stepping motor includes a set of permanent magnets 301 and 302 overlapped with each other through a disk magnetic material 303 fixed to a rotating shaft 311. The permanent magnets 301 and 302 are formed such that a plurality of magnetic poles formed by magnetization in the axial direction alternately become different magnetic poles in the peripheral direction. An inner tooth 305 radially protruding outward and an outer tooth 306 radially protruding inward are formed in each of the stators 307 and 308 which are opposed to the permanent magnets 301 and 302, respectively. The stators 307 and 308 are provided with excitation coils 309 and 310 for exciting the stators 307 and 308, respectively and positioned on both sides of the permanent magnets 301 and 302 such that the stators 307 and 308 are shifted from each other by an electrical angle of (½)π.
Here, a magnetic flux passes through a housing 317 surrounding the excitation coil 309, the outer tooth 306, the permanent magnet 301, the magnetic material 303, the permanent magnet 302, and the inner tooth 305 in order and then returns to the housing 317. The permanent magnet 301 is a magnet. When the inner tooth 305 and the outer tooth 306 which serve as an inlet and outlet of the magnetic flux are intended to oppose to the plane-magnetized magnets, the inner tooth 305 and the outer tooth 306 are actually on substantially the same plane. The magnetic material 303 serves as merely a back metal and is not excited by the excitation coils 309 and 310. Constituent elements required for the motor are (a) the housing 317 (also serving as a yoke), (b) the excitation coil 309, (c) the inner tooth 305 and the outer tooth 306 (magnet pole teeth), (d) the magnet (permanent magnet 301), (e) the magnetic material 303, (f) the magnet (permanent magnet 302), (g) the inner tooth 305 and the outer tooth 306 (magnet pole teeth), (h) the excitation coil 310, and (i) the housing 318 (also serving as a yoke), which are shown in order along the axial direction.
As described above, according to the stepping motor described in Japanese Patent Publication No. H06-083561, a large number of elements are located so as to overlap with the magnets having the surfaces perpendicular to the shaft in the axial direction. Therefore, a length of the stepping motor in the axial direction lengthens, so that it is hard to thin the stepping motor. In addition, the inner tooth and the outer tooth are provided in each of the stators and located such that the inner tooth and the outer tooth are shifted from each other by an electrical angle n in the case where a distance between the same magnetic poles in the peripheral direction of the magnets is given as one electrical angle. Therefore, when the number of magnetic poles of the stepping motor is intended to increase, a width of each of magnetic pole teeth and a gap between adjacent magnetic pole teeth become narrower, so that the difficulty in part processing increases and a physical strength reduces. Thus, the development of a stepping motor which can be thinned and is capable of dealing with an increase in pole has been desired.
FIGS. 22 and 23 show a two-phase stepping motor disclosed in Japanese Patent Publication No. H06-083564 as another example of the conventional technique. FIG. 22 is an exploded perspective view showing the two-phase stepping motor and FIG. 23 is a sectional view after assembly. The stepping motor includes a rotating shaft 403, a first permanent magnet 401, and a second permanent magnet 402. The first permanent magnet 401 and the second permanent magnet 402 are fixed to the rotating shaft 403 and magnetized in a thickness direction. In each of the first permanent magnet 401 and the second permanent magnet 402, different magnetic poles are alternately formed in a peripheral direction. A first stator 406 includes: a set of magnetic pole teeth 404 and 405 which are opposed to both sides of the magnetic poles of the first permanent magnet 401; and an excitation coil 410. A second stator 409 includes: a set of magnetic pole teeth 407 and 408 which are opposed to both surfaces of the magnetic poles of the second permanent magnet 402; and an excitation coil 411. The magnetic pole teeth 404 and 405 of the first stator 406 and the magnetic pole teeth 407 and 408 of the second stator 409 are positioned such that the first stator 406 and the second stator 409 are shifted from each other in the peripheral direction by an electrical angle of (½)π. When they are assumed to be constituent elements of a magnetic circuit, the magnetic circuit becomes a six-layer structure which includes the magnetic pole tooth 404 of the first stator 406, the first permanent magnet 401, the magnetic pole tooth 405, the magnetic pole tooth 408, the second permanent magnet 402, and the magnetic pole tooth 407 of the second stator 409. The number of air gaps between each of the stators and each of the magnets is two per phase, that is, four in total. Note that the stator is a stator.
As described above, the stepping motor described in Japanese Patent Publication No. H06-083564 is composed of a large number of parts provided in a multi-layer form. Therefore, a length of the stepping motor in the axial direction lengthens, so that it is hard to thin the stepping motor and a cost of parts increases. A large number of parts are provided in the multi-layer form and overlapped with one another in the axial direction. In addition, the number of air gaps between the stators and magnets, by which a magnetic characteristic is significantly influenced, is four, so that the parts cannot be easily assembled with high precision. It is necessary to pass the central rotating shaft through a large number of parts in order during assembly, so that an assembly time lengthens and it is likely to increase an assembly cost. The number of parts is large, so that it is likely to reduce assembly precision due to the accumulation of part processing precision. Thus, it is hard to construct a high performance stepping motor.
A stepping motor having a short shaft is disclosed as another conventional technique (Japanese Patent Application Laid-open No. H02-058035). FIG. 24 shows a structural example of the stepping motor provided as a driving source for an exposure quantity adjusting apparatus, which is disclosed in Japanese Patent Application Laid-open No. H02-058035. A surface of a magnet 505 which is perpendicular to an axial direction is divided in a peripheral direction and magnetized. Two coils 506 and 507 are located on both sides of the magnet 505. The magnet 505 and the coils 506 and 507 are sandwiched by two stators 508 and 509 in a shift direction. Therefore, it is possible to suitably incorporate such a thin stepping motor into the exposure quantity adjusting apparatus for a camera.
However, there are the following problems in the structural example. First, the pole teeth per phase of the stators 508 and 509 which are opposed to the magnet 505 correspond to about half of the entire circumference, so that it is possible to use only a magnetic flux equal to or smaller than half of an effective magnetic flux of the magnet. Therefore, it cannot be said that the above-mentioned structure is effective. Second, the coils are close to the stators in only upper and lower ends, so that the amount of leakage magnetic flux from the coils to the circumference is large. Therefore, it cannot be said that the above-mentioned structure is effective. Third, the pole teeth of the stators which are opposed to the magnet correspond to about half of the entire circumference as described above, so that a range in which a driving force is caused in each excited phase also corresponds to about half of the entire circumference. Therefore, when the driving force is converted into torque, an unnecessary force in a transverse direction is likely to cause, with the result that there are concerns with respect to vibration, noise, non-uniform rotation, and a reduction in positional precision in the stepping motor.
As described above, the conventional actuators and stepping motors have the following problems. The number of parts is large, thereby increasing a cost. It is hard to achieve slimming. High efficiency is hardly obtained. It is hard to deal with an increase in number of poles. Parts cannot be easily fit.